This invention relates to apparatus for monitoring the thickness of slag or ash in a furnace.
Coal typically contains silica, alumina and one or more of the oxides of iron, in addition to relatively small quantities of other metal oxides. These components are incombustible, and form slag or ash when the coal is burned. Slag is a fluid or solid agglomeration of the incombustible portion of burned coal, while ash is the same material in the form of fine particles that are not fused or agglomerated. A problem that results from burning coal in furnaces is the effect of the incombustible components of the coal on the operation of the furnace. This is particularly a problem when the coal is pulverized to improve combustion.
One requirement for efficient operation of a furnace is good heat exchange between the hot combustion products of coal and the water that is to be heated. When the incombustible portions of the coal, whether in the form of molten ash or solid particles, come into contact with the heat-exchange surfaces such as water walls and boiler tubes, the particles tend to adhere to the heat-exchange surfaces and interfere with heat transfer, creating slag or ash buildup. As a result of such slag or ash buildup, the capacity of the furnace is typically reduced. To maintain the capacity of the furnace, the heat-exchange surfaces are cleaned periodically by blowing steam or hot air through the furnace or by other suitable processes.
Slag or ash buildup is a particular problem in some oil or gas furnaces that are converted to burn coal, since oil and gas furnaces do not tend to collect as much slag or ash and their interiors are designed accordingly. The incombustible products of the combustion of coal tend to be abrasive, and in the cleaning process, the incombustibles erode internal furnace components of such coal furnaces. It is therefore preferable to clean the furnace of slag or ash accumulations only when it is necessary. However, the expense and loss of time incurred in measuring furnace slag or ash buildup by the visual internal inspection has created a need for apparatus which measures slag or ash buildup without internal visual inspection.
U.S. Pat. No. 4,552,098, entitled "Convection Section Ash Monitoring," discloses apparatus for monitoring the thickness of ash in furnaces. That patent teaches the use of radiation pyrometers to measure temperature drop as combustion products pass a bank of heat exchanger tubes. A fouling factor is calculated from the data generated by the pyrometers, and the fouling factor is a known or calculable function of the accumulation of incombustible material on the heat exchanger. Thus, the apparatus determines slag or ash buildup indirectly, in a manner which requires additional calculation. In addition, the radiation pyrometers of the '098 patent provide only an average indication of fouling.
In U.S. Pat. No. 4,511,371, the buildup of slag or ash in the combustor is monitored visually. This apparatus obviously requires human presence, which is not always desirable or possible. Thus, there is a need for apparatus for monitoring slag or ash buildup in furnaces without internal visual inspection which does not require processing of data or human inspection.